The Doctor and his companions Ben and Polly arrive in the TARDIS at the South Pole in the year 1986, near the Snowcap base. The base is supervising the mission of the Zeus IV spaceship, running a routine probe on the Earth's atmosphere.

Unusual readings on the spaceship's instruments lead to the discovery of a new planet suddenly approaching Earth. The spaceship begins to experience power losses, and Snowcap personnel begin arrangements to abort its mission.

Back on the base, the Doctor reveals what he knows about the tenth planet: it is Mondas, Earth's former sister planet and its inhabitants will soon be visiting Earth. True to his prediction, three robotic creatures land outside, killing the guards and disguising themselves in the dead men's furs to gain access.

While everyone is distracted by their efforts to land Zeus IV safely, the creatures are easily able to take over the base. The base personnel and Polly plead with the invaders to allow them to save the lives of the Zeus IV crew, but the creatures say that their lives are irrelevant to them. They explain that they are Cybermen, who were once like human beings, but gradually replaced their bodies with mechanical parts, including eliminating the "weakness" of emotion from their brains. The Cybermen allow the men to make contact with Zeus IV, but it is too late as the ship is dragged away by Mondas and explodes.

The Cybermen explain that Mondas is absorbing energy from Earth and will soon destroy it. They propose to take humans back to Mondas and turn them into Cybermen.

Ben, who has been imprisoned in the projection room after attempting to kill a Cyberman, rigs up the projector to blind incoming Cybermen, allowing him to steal his guard's weapon and kill him. Sneaking back into the Tracking Room, he hands the cyberweapon to General Cutler, the base commander, who kills the remaining two Cybermen. Cutler contacts Space Command HQ in Geneva and is informed that they have sent his son on a mission to rescue the doomed Zeus IV.

Cutler decides it is time to take the fight to the Cybermen and decides to use the powerful Z-bomb to destroy Mondas. Ben argues against using the bomb, saying that Mondas might destroy itself anyway when it absorbs too much energy. The chief scientist at Snowcap, Dr. Barclay, is also concerned, saying that the radiation caused by the exploding planet would cause great loss of life on Earth. Unswayed, Cutler orders Ben to be imprisoned in a cabin with the Doctor, who is unconscious and seemingly ill.

Barclay goes to Ben and tells him how to sabotage the rocket to prevent it from reaching Mondas, but Cutler notices Barclay's absence and catches Ben in the act. Cutler attempts to fire the Z-bomb, but the engines fail on the launchpad thanks to Ben's sabotage. Cutler, enraged, threatens to kill Ben, Barclay, and the Doctor, who has now regained consciousness. Before he can shoot the Doctor, Cutler is killed by the leader of a newly arrived squad of Cybermen.

The Cybermen insist that the rocket pointed at Mondas be dismantled. The Doctor suggests that it would be a good idea to go along with this, and tells the others to play for time, as Mondas cannot take much more energy now. The Cybermen take Polly back to their spaceship as a hostage.

As the Cybermen take over Space Command in Geneva, the Doctor realises that their plan is to destroy the Earth with the Z-bombs, thus saving Mondas. He manages to communicate this revelation to Ben and the others over the intercom before the Cybermen take him prisoner. In the radiation room, Ben surmises that the reason why they need to use humans to do this work rather than doing it themselves is that they are highly susceptible to radiation. Barclay suggests using the rods from the reactor chamber as a portable weapon against the Cybermen. This proves successful, allowing Ben, Barclay, and the others to regain control of the base. More Cybermen enter the Tracking Room, but just at that moment Mondas explodes, disabling all the remaining Cybermen.

The Doctor regenerates for the first time.

Cutler's son contacts the base from Zeus IV, telling them that his ship is now back to full power and Geneva tells Barclay that the Cyberman threat has ended. Meanwhile, Ben makes his way back to the Cybermen's ship to rescue the Doctor and Polly. The Doctor seems to deliberate as to whether "it's all over" before settling that it's "far from over" before making his way back to the TARDIS. As Ben and Polly hammer on the door, the controls operate of their own accord and the central column begins to rise and fall. The Doctor seizes enough energy to open the door and Ben and Polly finally get in. Barely conscious, the Doctor collapses to the floor, and before the astonished eyes of his companions, he becomes enveloped in a bright light. After the light dies down, the Doctor`s face is different and has the appearance of a younger man.

The events leading up to this story, from the Cybermen's perspective, are heard in the Big Finish audio story Spare Parts with the Fifth Doctor and Nyssa. The story deals with many aspects left unexplained in this story; such as how Mondas left the solar system and returned again. The Sixth Doctor serial, Attack of the Cybermen, takes place in 1985, a year before the events of The Tenth Planet. In that serial the Cybermen attempt to alter history so Mondas is not destroyed.

While the Doctor regenerates at the end of this story, the process is unnamed. In the subsequent serial, The Power of the Daleks, the Doctor states that he has been "renewed", implying a restoration of youth rather than a change of body. The concept was not called "regeneration" until Planet of the Spiders. Script editor Gerry Davis later stated that it was intended for the energy drain from Mondas to be the cause of the regeneration, but that it did not come across clearly on screen.

All four episodes of this story feature a specially designed graphics sequence used for the opening titles and closing credits. Designed by Bernard Lodge, they were intended to resemble a computer printout.[2] In the opening credits for the first episode, Kit Pedler is incorrectly identified as "Kitt Pedler". In the opening credits for the third episode, Gerry Davis is incorrectly identified as "Gerry Davies."

William Hartnell did not appear in the third episode.[3] On the Monday before the programme was due to be recorded, he sent a telegram to the production team informing them that he was too ill to work. Gerry Davis rewrote the script to explain the Doctor's absence (his sudden collapse) and gave his dialogue to other characters, most noticeably Ben. This was not as much of an interruption to the episode's production as it would seem, as all four episodes had been written so that Hartnell would have relatively little to do in case of just such an event. The original draft of episode 4 did not feature the Doctor regenerating at the end.[4]

The First Doctor's last words were originally scripted as something similar to "No... no, I simply will not give in!" Time was running short towards the end of production, and director Derek Martinus opted not to record the line, wanting to ensure that the regeneration sequence was recorded as well as possible. As a result, the First Doctor's last words were simply "Ah! Yes. Thank you. That's good, keep warm."

Patrick Troughton appears in the final episode, uncredited, as the Second Doctor. William Hartnell would reprise the role of the First Doctor on only one occasion: the tenth anniversary serial The Three Doctors, although a photograph of him would be seen briefly as Troughton first looks into a mirror at the start of Troughton's first serial, The Power of the Daleks; the first episode begins with a shot of Troughton lying on the TARDIS floor. A clip of Hartnell from part 6 of The Dalek Invasion of Earth was used at the beginning of the 20th anniversary special, The Five Doctors (where Richard Hurndall would play the First Doctor during most of the episode). A clip of Hartnell from The Tenth Planet was used in Earthshock, when the Cybermen review the Doctor's change of appearance.

The last episode of this serial is missing. It is possibly the most sought-after of the missing episodes, because it contains the historic first regeneration scene (even though a low-quality, truncated copy of this sequence survives and is held in the BBC Archives), and also because it is William Hartnell's final episode. In fact, it is included in a list of the ten most wanted missing programmes[citation needed] alongside the BBC studio footage from the Apollo 11 landings (which is currently held only in soundtrack form).

Popular myth has it that the only surviving telerecording copy of the fourth episode was lost when loaned out to the children's programme Blue Peter in 1973 when they wished to use a clip from it in a feature on the tenth anniversary of Doctor Who.[5] Although a print of The Daleks' Master Plan Episode 4 ("The Traitors") was loaned to Blue Peter and not returned to the BBC Film Library, there was never a copy of The Tenth Planet Episode 4 there to have been loaned. Another department – BBC Enterprises – was still offering all four episodes for sale to foreign broadcasters until the end of the following year and would not, in any case, have loaned out master negatives.

In 1992, a man named Roger K. Barrett (later revealed to be an alias; it being based on the real name of Syd Barrett) claimed to have a videotape recording of Episode 4 of this story, and offered to sell it to the BBC for £500. Before this was revealed as a hoax, the BBC produced a special introduction for an intended VHS release of the story, hosted by Michael Craze, two versions of which were filmed: one explaining that Episode 4 was still missing, the other introducing the story as if it were complete. A documentary called "Missing in Action", made in 1993 and narrated by Nicholas Courtney, also mentions the hoax.

For the 2013 DVD release, episode four was animated by Planet 55 Studios.[6]

In 2009, Patrick Mulkern of Radio Times found the original Cybermen design like "usherettes from some kinky, futuristic moviehouse", but praised the character of Cutler and Hartnell's Doctor.[10] Den of Geek named the cliffhanger of Episode 4 as one of the programme's ten "classic" cliffhangers.[11] Alasdair Wilkins of io9 described it as "a very solid, at times excellent story" and noted "The Cybermen have possibly been more intimidating in other stories, but they have never been creepier than they are here". He named it the fourth best regeneration and regeneration story.[12]DVD Talk's John Sinnott gave the story four and a half out of five stars. He praised Hartnell's performance and the Cybermen.[13] Ian Berriman of SFX was more mixed, giving the serial three out of five stars. He praised the Cybermen and the "palpable tension", but felt that the regeneration was tacked on and not enough background was given to make Mondas believable.[14]

A novelisation of this serial, written by Gerry Davis, was published by Target Books in February 1976. It was the first Hartnell-era serial novelisation to be commissioned by Target, and the first new adaptation of a Hartnell adventure to be published in nearly ten years.

The novelisation largely follows the original script and so places the action in the year 2000 as well as restoring the Doctor to the third episode. Also, in the first scene in which the Doctor, Ben and Polly appear (in the TARDIS), the Doctor is beginning to show signs of his failing health; sometimes mistakenly addressing Ben and Polly as "Ian" and "Barbara", thereby revealing signs that all is not as it should be. Also, the regeneration of the Doctor occurs in the TARDIS differently. The Doctor uses what appears to be a rejuvenation chamber that assists him in his regeneration.

The story was released on VHS in the UK in 2000 from BBC Video, with the fourth episode reconstructed by the Doctor Who Restoration Team using still photos, existing clips and the surviving audio soundtrack. This release was a double-tape set entitled "Doctor Who: The Cybermen Box Set: The Tenth Planet and Attack of the Cybermen". In the U.S. and Canada both stories were released individually in 2001. The existing clips from the missing final episode – 8 mm film recordings made by fans and a 16mm film clip of the regeneration (from a 1973 edition of Blue Peter) – were included in the DVD release Lost in Time in 2004. The only surviving clip of the regeneration was also released as a special feature on the DVD releases for The Three Doctors and Castrovalva.

The story was released on DVD on 14 October 2013,[15] with the missing fourth episode animated[16][17] along with additional extra features including the original reconstruction of episode four from the 2000 VHS Release included as an extra and a special "Frozen Out" documentary on the making of the story.

The "Regenerations" box set, released on 24 June 2013,[18] contains The Tenth Planet (including the newly animated fourth episode). The serial was released with no special features.[citation needed]

The soundtracks for The Tenth Planet and The Invasion, put together from fan-made recordings, along with a bonus disc, The Origins of the Cybermen, an audio essay by Cyberman actor David Banks, were released on CD in a collector's tin called Doctor Who: Cybermen.